This invention relates generally to foot pressure sensors and more particularly to a system that may be employed by orthopedic surgeons, their patients, physical therapists, athletes, coaches, trainers, and others to measure forces applied to the foot.
Controlling the weight placed on an affected leg while moving about can prevent injury and speed recovery of patients recovering from certain trauma or orthopedic procedures. A person is able to perceive a changing relative weight on a foot, but cannot reliably sense how much constant weight is actually being loaded. That is, an individual's own senses cannot reliably detect how much actual constant weight he or she is placing on a foot. This essential weight information must therefore be provided by some type of external or artificial device.
Forces applied to the foot are not always evenly distributed over the entire surface or sole of the foot. They are usually, but not always, concentrated at the heel or ball of the foot, and they are also usually not applied to these two areas at the same time. A typical walking or running foot strike first loads the heel and then the ball of the foot. In other situations, such as when performing an athletic maneuver, the weight placed on the foot will shift in different patterns. The specific location on the sole of the foot where forces are present, the times at which those forces occur, and the magnitude of those forces are of interest in a number of situations.
Many devices have been designed to measure forces applied to the foot. These include devices for measuring applied forces that are coincident with the position of the leg and body during walking or running, for measuring peak pressure to alert sensory impaired individuals such as diabetics, for measuring the total force applied to the foot, and for measuring forces applied to certain areas of the foot for various purposes.
Such prior art foot force measurement devices are typically heavy and cumbersome and, thus, act to restrict the wearer's movements. Devices which employ an electronic package strapped to the wearer's leg above the ankle and electrically connected to a foot sensor require extra effort when attaching or removing the device. Although this extra effort may seem insignificant, it is nevertheless an inconvenience that may prompt wearer's of such devices to neglect their use. Further, these bulky prior art devices chafe the wearer's skin, are prone to snagging, and are subject to failure resulting from stress on the electrical cable that connects the electronic package to the foot sensor. Other prior art devices employ a stationary foot sensor pad and are therefore only suited for use in a laboratory.
All of the known foot force measurement devices function to convert mechanical force into a suitable signal medium, usually electrical signals. Consequently, the prior art can be conveniently categorized according to the type of sensor used to convert changes in mechanical force to changes in electrical signals. Accordingly, these types of prior art sensors include pneumatic or hydraulic fluid activated switches, strain gauge sensors that respond to mechanical deformation, single direct electronic force sensors, multiple direct electronic force sensors with random spacing, and multiple direct electronic force sensors with regular spacing.
Pneumatic and hydraulic sensors are prone to produce inaccurate readings because changes in internal pressure are not necessarily proportional to changes in force. An accurate hydraulic sensor can be constructed if the fluid is displaced from a containing structure which itself, not the fluid, supports the applied forces, and the force required to activate a switch or sensor is very small compared to the force being measured. Exemplary of such prior art sensors are those taught in German patent 3631-923-A to Ernst and in U.S. Pat. No. 3,974,491 to Sipe. Another way to construct an accurate fluid sensor is to use rigid plates which limit the applied force to a constant area of fluid contact, as taught in U.S. Pat. No. 3,791,375 to Pfeiffer. In such sensors, the fluid pressure is proportional to the applied force. However, these sensors are disadvantageous due their bulkiness and weight.
The sensors which measure mechanical deformation of structural elements supporting the wearer's foot by use of electrical wire or ribbon type strain gauges accurately measure weight, but they are also disadvantageous because of their bulk and weight. Exemplary of these prior art sensors are those taught by Donald Endicott et al., "Leg Load Warning System for the Orthopedically Handicapped," Medical and Biological Engineering (May, 1974) S. Miyazaki and H. Iwakura, "Foot-Force Measuring Device for Clinical Assessment of Pathological Gait," Medical and Biological Engineering and Computing (July, 1978), and G. A. Spolek and F. G. Lippert, "An Instrumented Shoe-A Portable Force Measuring Device," J. Biomechanics (Great Britain, June, 1976).
All of the below-mentioned direct electronic force sensor devices are essentially identical to a type of microphone used in telephones. In this prior art device, the acoustic force compresses a partially conductive material (carbon granules) and therelay reduces its electrical resistance. A force sensor based on this concept can be constructed by forming thin electrodes on two plastic films and then covering the electrodes with a thin layer of suitable partially conductive material. When the two coated films are positioned sot that the two layers of partially conductive material are in contact, the electrical resistance through the sensor from electrode to electrode varies inversely with a compressing force on the sensor. This is because as the coating is pressed with greater force, the electrical current path becomes broader due to increased area of contact. This effect occurs with any conductive material, but materials useful as active elements in practical force sensors must have a relatively high electrical resistance. Such thin film electronic force sensors have the advantages of simplicity, compactness, and light weight.
Single direct electronic force sensors, such as that taught in German patent 2656-864, cannot be corrected for variations in response from one part of the sensor to another. They cannot separately measure forces on different parts of the foot. It is also possible that simple bending of this type of sensor will produce a signal inappropriately indicating an applied force.
The multiple direct electronic force sensor system taught in U.S. Pat. No. 4,813,436 to Au measures forces only where the individual sensors are attached to the foot. If the measurements are used to compute total force applied to the foot and are variously spaced, the contributory area of each sensor must be used in the necessary computation of the total force applied. This prior art system is disadvantageous in that the relative position of each sensor must be separately determined for each person on which the sensor is used.
This problem is solved by the multiple direct electronic force sensors taught in U.S. Pat. Nos. 4,734,034 and 4,856,993 to Maness et al. and U.S. Pat. No. 5,033,291 to Podoloff et al. in which the sensors are regularly spaced. Since the relative position of each sensor is fixed, a mathematical description of the location of each sensor can easily be made part of a permanent computer database. These sensor arrays are very thin and very light weight, but they cannot conform to a compound curved surface without wrinkling. Such wrinkled or folded thin film sensor arrays will produce erroneous results. For example, if the sensor array is folded so that two separate sensors are positioned one above the other, they both measure the same force. This is an obvious error. A folded sensor array may produce an electrical signal from the folding alone, another obvious error. Folding or wrinkling also subjects the sensor array to severe fatigue, stress, which can lead to early and sudden failure.
A practical sensor array preferably includes-several hundred individual force sensors. The problem of electrically connecting this number of individual sensors to data acquisition circuitry is a significant one. A simple connection scheme would require as many electrical connections to the circuitry as sensors, plus one common connection. However, the circuitry described by John A. Purbrick, "A Force Transducer Employing Conductive Silicon Rubber," First Robot Vision and Sensors Conferences, Statford-on-Avon, England (April, 1981), makes a more efficient connection method possible. This prior art circuit can measure current through individual force sensors in a two-vector array of rows and columns with a top electrode of all sensors in each column commonly connected and a bottom electrode of all the sensors in each row likewise commonly connected. If such an array has equal numbers of columns and rows, the number of connections to circuitry is reduced to twice the square root of the number of sensors. The Purbrick method is not as efficient when employing other ratios of columns to rows of sensors, but it nevertheless represents the most efficient method known for interconnecting the sensor array of the present invention. A similar method is taught by W. D. Hillis, "Active Touch Sensing," Artificial Intelligence Laboratory Memo 629, M.I.T. (April, 1981).
It is therefore a principal object of the present invention to provide a self-contained force measurement system for continuously sampling the magnitude and distribution of forces that are present on the bottom of one or both feet of the user when engaged in various activities.
It is a further object of the present invention to provide such a force measurement system that does not unduly impede walking, running or other ordinary movements of the user.
It is a further object of the present invention to provide such a force measurement system that is self-contained for use away from a laboratory or clinic and that is capable of annunciating appropriate weight bearing conditions, of generating computer data for future reference and analysis, and of transmitting this computer data by wireless means to remotely located equipment.
These and other objects of the present invention provide a force measurement system useful to the medical patient, attendant physician, physical therapist, prosthetist, orthotist, podiatrist, athlete, sports trainer, coach or research professional interested in carefully studying or managing a wide range of problems of the human lower extremity.